There exists a class of minerals that are formed from sheets or layers that have an hexagonal crystal structure and that are referred to herein as hexagonal layered minerals. Such minerals include graphite, molybdenum disulfide, tungsten disulfide (diselenide), hexagonal boron nitride, vanadium pentoxide, vanadium X oxides (e.g., vanadium selenium oxide), and like minerals.
Graphite is perhaps the best known of the hexagonal layered minerals because it exhibits basal cleavage, good electrical and heat conductivity, refractoriness and resistance to acids. It is an important component in many technologies, such as leading-edge alternative energy solutions, including batteries and hydrogen fuel cells. It is also used in producing electrodes and brushes for electric motors. Graphite is also a key component in metallurgic and refractory materials, and is used as a release agent in molds, dies and form linings when making metal parts and castings.
Graphite is obtained by mining graphite-rich ore (i.e., graphite rock) and milling it down to the consistency of sand to allow the graphite to be removed. The milled material is then put through a series of flotation processes to extract the graphite.
A variant of graphite is expanded graphite, which is graphite that has been treated such that the interlayer distance between the individual crystal planes is expanded beyond the usual van der Waals distances. Expanded graphite has a much higher energy and gas storage capacity than ordinary graphite.
Conventionally, graphite rock has to be milled before it can be processed as expanded graphite. To produce expanded graphite using prior art techniques, the milled graphite flakes are immersed in concentrated acids. All these processes are energy intensive, and the acid intercalation process creates many defects in the graphite, which adversely affect the graphite's electrical conductivity. Working with acids is also generally not preferred because they are caustic and difficult to handle.
Graphene is a derivative of graphite and comprises a two dimensional sheet of hexagonally arranged atomic carbon with very attractive physical, optical and mechanical properties, including high charge carrier mobility, record thermal conductivity and stiffness.
Few-layer graphene (FLG), which can be derived from the exfoliation of graphite or graphite oxide, exhibits better dispersion properties and therefore can form more homogeneous blends or composites with other materials than graphite can. It is expected that enhanced performance can be obtained where FLG substitutes for graphite flakes as the key component in coatings, metallurgy or refractories.
What is needed are efficient processes for forming expanded hexagonal layered minerals and their derivatives without the need to perform complex and potentially hazardous processing steps.